There are various known power tools that may be operated using an electric motor. In some cases these power tools are provided with EC motors that are brushless, i.e., the rotor is provided with permanent magnets that rotate in a magnetic field generated by stator coils. The rotational speed of the EC motor is usually controlled via the applied motor voltage.
The motor voltage is usually applied by pulse width modulation of the power transistors of the drive unit. In doing so, the motor is commutated as a function of fixed rotor positions. The EC motor is usually designed for the maximum rotational speed of the power tool. Because this is usually associated with a lower number of required windings, it results in a comparatively low torque in relation to the stator current. This current is determined by the limiting values of the power electronics, the motor and/or the battery. The torque is thus also limited to this maximum value over the entire rotational speed range.
This behavior is unfavorable for use of such EC motors in power tools. In this situation, it is often necessary for a high torque to be available at a low rotational speed and for a low torque to be available at a high rotational speed. With today's power tools, this is achieved by providing reversible gears with which a high torque at a high rotational speed is converted into a lower rotational speed of the tool to achieve a high torque at a lower rotational speed. However, it is complex and expensive to provide a reversible gear, which also results in friction losses that increase the power tools' power consumption.
In the past, power tools have been designed for a high rotational speed, so the stator coils have a low ohmic resistance and a lower inductance, which may result in a relatively high current when the coils are short-circuited, and consequently the motor or the electronics may be damaged. In the event of a fault or an inadmissible operating state, which may occur, for example, when a battery is removed from a battery-operated power tool when the motor is rotating, the motor and/or the electronics may be destroyed due to short circuiting of the motor. For this reason, measures are usually provided to prevent damage to the motor.
An object of the present invention is to provide an improved power tool that meets the requirements of a high torque at a lower rotational speed and a lower torque at a high rotational speed without using a reversible gear and whereby the power tool is more resistant to damage due to faults or inadmissible operating states.